Run Hugo build test on the correct Docs branch

What does this MR do?

Following on from: gitlab!190245 (merged), we want to be able to use a version of docs-gitlab-com that corresponds with the branch of GitLab Runner we're running on, not necessarily main. main on docs-gitlab-com can become backwards-incompatible with older GitLab Runner branches.

Output of job:

• When run on branch that targets main: https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/9979856370.
• When run on branch that targets a stable branch: https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/9979958094 (tested through !5557 (closed)).

Relates to: gitlab-org/technical-writing/docs-gitlab-com#322 (closed).